Melt polycarbonate is produced by the melt transesterification reaction of a dihydroxy compound (e.g. bisphenol A) and a diaryl carbonate (e.g. diphenyl carbonate) in the presence of a melt transesterification catalyst. Polymerization takes place in a series of reactors where operating conditions such as temperature and pressure are controlled so that the reaction byproduct, phenol, is removed from reaction components thereby driving the reaction by equilibrium displacement.
Two classes of catalysts, alpha and beta catalysts, are used in melt transesterification processes due in part to the temperature profile of the melt reaction process. Organic catalysts (i.e. beta catalysts) like tetrabutylphosphonium acetate or tetramethylammonium hydroxide are effective at the early formulation and oligomerization stages where temperature is below 200° C. Many beta catalysts however are volatile and can degrade at elevated temperature and reduced pressure leading to a loss of the catalysts from the reaction mixture. Due to the thermal degradation and volatility loss of the beta catalyst, an alkali or alkaline earth metal type catalyst (i.e. an alpha catalyst) is used to catalyze the reaction at higher temperature and reduced pressure. However, certain types of beta catalysts are stable along the polymerization process conditions (including the high temperatures and reduced pressures) and can be used solely or in combination with an alpha catalyst.
In a typical melt transesterification production facility all desired reaction components including melt transesterification catalyst(s), dihydroxy compound(s), and diaryl carbonate(s) are combined and mixed in a formulation and/or oligomerization tank(s) to form a reaction mixture suitable for continued polymerization throughout the balance of the polymerization system. After the reaction mixture is formed it is introduced to a series of polymerization reactors operating under melt polymerization conditions sufficient to build at least molecular weight to produce polycarbonate within a desired specification.
With respect to producing a product polycarbonate with desired specifications, the selection of melt transesterification catalysts as well as adjustment of process conditions allow for the production of polycarbonate resins that are differentiated by, among other things, molecular weight, branching, and terminal endgroups. By adjusting these variables different polymer grades can be produced using the same dihydroxy and carbonate monomer starting materials. In other words, adjusting the front end reaction mixture and/or the reaction system set points (e.g. flow rates, torques, reactor temperatures and/or pressures) provides the ability to produce polycarbonate having specific characteristics. This is an energy and equipment intensive process, which is further exacerbated when producing high branch and/or high molecular weight product polycarbonates; high branched and/or high molecular weight polycarbonate production typically require higher energy inputs along the system as the viscosity builds as the reaction mixture moves through the reactor system. Furthermore, it is noted that when a new grade of polymer is to be produced, the transition materials within the reactor system are purged and discarded until the new product polycarbonate meets desired specifications. This leads to wasted material, increased cost, and inefficiencies in the process.
Based upon the foregoing, improvements in the melt polycarbonate production process are desired to tackle the above-mentioned inefficiencies. In particular it would be most desirable to have the ability to produce a broad range of polycarbonate grades with superior qualities, including color, while minimizing process equipment, energy input, process steps, and wasted transition material. The present disclosure addresses these inefficiencies.